This application relates to a system and method for improving the performance of a hybrid power supply apparatus comprising a power generating device, such as a fuel cell system, and an energy storage device, such as a battery. The purpose of the invention is to provide an equalization charge to the battery from a source other than the power generating device when the battery achieves a predetermined state of charge condition, thereby avoiding the need to operate the power generating device in a low power output mode.
Hybrid power supply systems comprising a power generating device and an energy storage device are well known in the prior art. In recent years interest has grown in using fuel cells as power generating devices in such hybrid systems. The fuel cell is used to charge a storage battery which in turn supplies power to a load on an xe2x80x9con-demandxe2x80x9d basis. Alternatively, the fuel cell and the battery may jointly supply power to the load depending upon the power requirements.
Many fuel cell systems include fuels processors such as reformers for converting conventional fuels to hydrogen-enriched gas for processing by the fuel cell. In general, the combination of a fuel cell and a reformer makes it difficult for the power generating device to respond quickly to variations in external load since the response time of the reformer is slow. This is particularly the case for loads that fluctuate substantially over time. For example, electric lift vehicles have a pattern of power usage or xe2x80x9cduty cyclexe2x80x9d which is characterized by loads which fluctuate substantially during the course of a work shift. Hybrid power supply systems offer several advantages in such applications. The addition of a charged energy storage means enables the hybrid system to respond quickly to power demand surges, while maintaining the advantages of a fuel cell system, including extended operating times, low emissions and the flexibility to utilize many readily available fuels. Further, in hybrid systems the size of the power generating device may be minimized to enhance system efficiency and reduce cost.
Hybrid power supply systems are known in the prior art for use in applications subject to sudden load fluctuations. U.S. Pat. No. 4,883,724, Yamamoto, issued Nov. 28, 1989 relates to a control unit for a fuel cell generating system which varies the output of the fuel cell depending upon the state of charge of the battery. In particular, a DC/DC converter is connected between the output of the fuel cell and the battery and is responsive to a control signal produced by a controller. The purpose of the Yamamoto invention is to ensure that the storage battery is charged for recovery within the shortest possible time to reach a target remaining charge capacity under charging conditions that do not cause deterioration of performance of the battery. When the charged quantity of the battery is recovered to the target value, the controller lowers the output of the fuel cell. In the case of no external load, such as during extended interruptions in the operation of the lift truck, the fuel cell is controlled to shut-down after the storage battery is fully charged.
One limitation of the Yamamoto system is that the control algorithm is designed for prolonging the useful life of the storage battery rather than the fuel cell. By varying the fuel cell output to charge the storage battery for recovery within the shortest possible time, the long-term performance of the fuel cell is compromised. For example, frequent changes in fuel cell power output degrade the performance and lifetime of the fuel cell. Further, depending upon the state of charge of the battery, the fuel cell (and hence the fuel processor) may operate for extended periods in a low power output mode which is undesirable.
U.S. Pat. No. 4,839,574, Takabayashi, also discloses a generator system utilizing a fuel cell and reformer. Depending upon the state of charge of the battery the output of the fuel cell may be adjusted in a stepwise fashion. In the Takabayashi system the amount of raw material supplied to the reformer is maintained constant within a range of charged energy to ensure stable operation of the reformer. However, depending upon the state of charge of the battery, the fuel cell and the reformer may once again operate for extended periods in a low output mode.
Typically, using sealed lead acid batteries as an example, a constant voltage charge method is the preferred means for charging the battery cells. Under a constant voltage regime inrush currents are limited by the internal resistance of the battery. Thus, when the battery is in a low state of charge and the internal resistance is low, inrush currents can be very large and energy can be restored to the battery very quickly. As the battery becomes charged, it reaches a transition point where a sudden rise in internal resistance occurs and, under constant voltage, the battery will accept less and less current. This self-regulating effect prevents overcharging of the battery, leading to longer battery lifetimes. Typically such constant voltage charge regimes are conducted at the xe2x80x9cfloatxe2x80x9d or xe2x80x9cequalizationxe2x80x9d voltage which is the recommended voltage at which the batteries can be maintained at high states of charge.
Similarly, other advanced batteries such as Nickel Metal Hydride (NiMH) batteries may be charged at higher currents in a constant voltage or constant current regime until a transition point occurs. After this transition a low current equalization charge is required to return the complete capacity of the battery and to ensure that the individual cells within the battery are brought to an approximately equal charge state.
Most of the charging and discharging of the battery is done in the xe2x80x9cbulk regionxe2x80x9d below the transition point. Once the battery reaches the transition point or charge threshold (e.g. at approximately 80% of its total charge capacity), most prior art systems step down the output of the fuel cell and operate the fuel cell in a low power mode until the battery is fully charged (assuming the absence of an external load). In other words, a reduced current equalization charge is provided by the fuel cell to the battery in the xe2x80x9cabsorptionxe2x80x9d and xe2x80x9cfloatxe2x80x9d regions of the battery above the transition point. Although it is necessary to return the battery to a fully charged state to ensure sufficient capacity to meet future load demands, it is not desirable to operate the fuel cell and fuel processor in low power output modes for extended periods of time to provide such an equalization charge.
A need has therefore arisen for a system and method for providing an equalization charge to a storage battery in a hybrid system from a source other than the fuel cell once the battery achieves a predetermined state of charge condition, such as a threshold state of charge for a predetermined period of time.
In accordance with the invention, a hybrid power supply system for supplying electrical power to a load is disclosed. The system includes a power generating device, such as a fuel cell system; a first energy storage device chargeable by the power generating device and electrically connectable to the load; a first detector for measuring the state of charge of the first energy storage device; a second energy storage device chargeable by the power generating device and electrically connectable to the first energy storage device; and a controller for receiving input from the first detector. The controller causes the second energy storage device to provide an equalization current to the first energy storage device when a first predetermined state of charge condition is detected by the controller. For example, the first predetermined state of charge condition may occur when the measured state of charge of the first energy storage device exceeds a first threshold value for a predetermined amount of time.
The power generating device is operable in alternating charging and shut-down or non-charging modes. The controller causes the power generating device to switch from the charging mode to the shut-down mode when the first predetermined state of charge condition is detected. The second energy storage device delivers the equalization charge to the first energy storage device while the power generating device is in the shut-down mode until the first energy storage device is fully charged. The invention avoids the need to operate the power generating device in a low power output mode for extended periods of time.
The system may also comprise a second detector for measuring the state of charge of the second energy storage device. The controller causes the power generating device to power back on when a second predetermined state of charge condition is detected. The second predetermined state of charge condition may occur, for example, when the measured state of charge of one or both of the first and second energy storage devices falls below a second threshold value for a predetermined amount of time.
The second energy storage device may also be electrically connectable to the load. In one aspect of the invention, the second energy storage device delivers current to the load when a third predetermined state of charge condition has occurred. For example, the third predetermined state of charge condition may occur when the measured state of charge of the first energy storage device is a state of charge approximately equivalent to the measured state of charge of the second energy storage device. This feature prevents over-discharging of the first energy storage device and increases the stored energy available to the load.
As indicated above, the power generating device may comprise a fuel cell. The system also preferably includes a DC/DC power converter connected between the fuel cell and the first and second energy storage devices for controlling the current delivered to the energy storage devices. The first and second energy storage devices may comprise, for example, batteries or supercapacitors.
The invention also relates to a method of operating a hybrid power supply apparatus including a power generating device adjustable between charging and shut-down modes and further including first and second energy storage devices chargeable by the power generating device. At least one of the first and second energy storage devices is connectable to a load for delivering electrical power thereto. The method includes the steps of (a) operating the power generating device in the charging mode to deliver charging current to the first and second energy storage devices and to the load; (b) repeatedly measuring the state of charge of the first energy storage device; (c) adjusting the power generating device from the charging mode to the shut-down mode when the measured state of charge of the first energy storage device satisfies a first predetermined state of charge condition; and (d) providing an equalization current from the second energy storage device to the first energy storage device while the power generating device is in the shut-down mode until the measured state of charge of the first energy storage device reaches approximately its full storage capacity. The first predetermined state of charge condition may occur, for example, when the state of charge of the first energy storage device exceeds a first threshold value for a predetermined length of time, the first threshold value being a percentage of the full storage capacity of the first energy storage device.
The method may further include the step of adjusting the power generating device from the shut-down mode to the charging mode when the measured state of charge of the first energy storage device and/or the second energy storage device satisfies a second state of charge condition, for example when one of such states of charge is below a second threshold value for a predetermined period of time.
The method may also optionally include the step of electrically connecting both of the first and second energy storage devices to the load for jointly delivering electrical power thereto. In another embodiment, the power generating device may be adjusted from the charging mode to the shut-down mode when the combined states of charge of the first and second energy storage devices exceed a predetermined amount.
In one aspect of the invention the apparatus is operable in successive alternating first and second operating cycles. In the first operating cycle, the second energy storage device delivers an equalization current to the first energy storage device when the power generating device is in the shut-down mode. In the second operating cycle the first energy storage device delivers an equalization current to the second energy storage device when the power generating device is in the shut-down mode. Hence both storage devices are periodically equalized and returned to full capacity and the fuel cell is not required to operate in a low power output regime.